Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, indium, antimony, tellurium, aluminum, etc., with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam.
Heretofore, various systems in which the line-emitting element is presented for irradiation/electric discharge have been disclosed. Many diverse forms and states have been attempted, to include, presenting the element in pure form, e.g. pure metal, presenting the element as a compound, e.g. a salt, or in a solution, e.g. dissolved in a solvent such as water. Moreover, systems have been disclosed in which the line-emitting substance is presented as a liquid, gas, vapor and/or solid and can be in the form of a droplet, stream, moving tape, aerosol, particles in a liquid stream, gas jet, etc.
In addition to the large variation in source materials/material forms described above, a plethora of techniques have been developed to create a plasma of the source material. For example, a number of discharge plasma production (DPP) techniques have been developed such as capillary discharge, z-pinch, dense plasma focus, electrodeless z-pinch, star-pinch, etc. In a similar manner, for laser produced plasma systems, there are a number of choices available such as laser type, wavelength, pulse energy, etc.
From the above discussion, it is apparent that there are a relatively large number of possible EUV light source configurations. With this in mind, a number of factors warrant consideration when designing an EUV light source, especially a light source intended for high volume, production photolithography. One such factor is conversion efficiency, C, which is typically defined as the ratio of output EUV power, B, to input power, A, (i.e. the power required to operate a drive laser or create a gas discharge), C=B/A. For at least some LPP setups, conversion efficiency is a function of the type of laser used, the nature of the line emitting element and the coupling between laser beam and target. This coupling, in turn, may depend on the composition of the target, with some target materials allowing deep penetration of the laser beam into the target resulting in relatively high conversion efficiency.
Another factor that is often considered when designing a high volume EUV light source is the generation and mitigation of debris which may damage EUV light source optics such as a laser input window, collector mirror and/or metrology equipment. Thus, for at least some source materials, the production of a plasma may also generate undesirable by-products in the plasma chamber (e.g. debris) which can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements. This debris can include out-of-band photons, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of source material. This debris may also include chamber material from secondary sputtering and for the case of electric discharge type systems, electrode material. For this reason, it is often desirable to employ one or more techniques to minimize the types, relative amounts and total amount of debris formed for a given EUV output power. When the target size, e.g. droplet diameter, and/or target makeup, e.g. chemistry, are chosen to minimize debris, the targets are sometimes referred to as so-called “mass limited” targets.
The high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them. Thus, debris reduction and/or suitable techniques to reduce the impact of debris may need to be considered in the design of a high volume EUV light source.
Another factor that may be considered when selecting a source material is the temperature at which the source material may need to be processed. For example, pure lithium and tin both have relatively high melting points, which in some cases may forbid their practical use in applications in which piezo-electric materials are employed to produce a uniform stream of source material droplets. Other factors that may influence the choice of a target material include the materials toxicity and the materials compatibility (e.g. corrosiveness, etc.) with the source material dispenser.
Specific examples of EUV light source configurations include U.S. Pat. No. 6,831,963 which discloses the use of tin bromides in solution and at room temperature to produce a debris-free EUV emission, and U.S. Patent application No. 2005/0167617 which discloses the use of tin bromides due to their higher vapor pressure as compared with pure tin, and primarily describes the use of tin halogenide vapor as a source material for a gas discharge EUV light source. Another example of EUV light source configuration is disclosed in an article by Guenther Derra et al., entitled, Tin Deliver Systems for Gas Discharge Sources, that was presented at an SPIE EUV source workshop on Feb. 27, 2005. In the Derra et al. article, the use of stannane gas (SnH4) in a gas discharge EUV light source is disclosed. Also, PCT application WO2004/104707 to Zink et al. entitled, Method and Device for Cleaning at least One Optical Component, discloses an EUV irradiation device in which an optical component becomes contaminated due to an inorganic substance introduced by a radiation source and the device includes a supply device to introduce a reactant for the purpose of removing the deposits. WO2004/104707 discloses that the radiation source may include tin and the reactant may include a halogen or halogen compound. Still, despite these disclosures, a suitable light source configuration for high volume EUV lithography has yet to be developed.
With the above in mind, Applicants disclose a laser produced plasma EUV light source, and corresponding methods of use.